Digital radio, or digital audio broadcasting (DAB), is a method for transmitting digital quality audio signals to digital radio receivers. In-Band On-Channel (IBOC) transmission is a broadcasting technology characterized by the transmission of digital signals in the existing AM and FM spectrum and according to existing station assignments. A digital radio transmission scheme that uses IBOC transmission can be expected to deliver compact disc quality sound at the existing FM radio dial positions. Similarly, a digital radio system that uses IBOC transmission for AM radio can be expected to deliver FM quality sound.
IBOC digital radio transmission involves generating a digitally modulated signal that will exist on the same frequency as an existing analog station. Several digital modulation schemes are available. For example, audio sub-band digital coding techniques can be used to compress the digital content of the signal to fit within the frequency mask of each station frequency. Audio coding algorithms or schemes can also be based on acoustic measurements as a method for identifying those portions of the audio transmission that are inaudible to the is human ear and need not be transmitted As a result, the coding algorithms can sample the signal and delete the inaudible portion, thereby permitting significant audio compression and conservation of bandwidth without degrading audio quality. Because of the data compression of the coding algorithms, the compressed signal can occupy the available bandwidth of the AM and FM spectrums. In this manner, the available bandwidth can be used as a data channel.
A plot of a frequency mask as a function of power (dB) versus frequency (Hz) is wider at the bottom than it is at the top. Because a digital signal can be transmitted at lower power than an analog signal, the digital signal can occupy the wider, bottom part of the frequency mask without interfering with adjacent stations or signals. For example, the FM IBOC digital radio signal can occupy the sidelobes of the FM mask and the analog FM signal can occupy the frequency space between these sidelobes. In the case of AM radio, the AM IBOC signal can employ frequency separation and quadrature modulation to avoid interference with the analog AM signal.
Advantages of digital transmission for audio include better improved quality, less noise, and, a wider dynamic range, as compared with existing AM and FM radio. In addition to improved audio quality, IBOC digital audio broadcasting also provides for the transmission of data. Although FM subcarriers are now used to deliver data for many applications, IBOC digital audio broadcasting subsystems can accommodate larger amounts of data with greater reliability. Furthermore, prior to IBOC digital audio broadcasting, there has been no such capability in the AM band. Since the data may be audio or video, potential applications for data include station data such as call sign, format, artists and song titles, as well as music videos, images, news, financial and stock market data, paging, e-mail, dispatching, computer communications, and networking However, digital radio still possesses some of the shortcomings of traditional AM/FM radio. The user of a digital radio receiver can listen to or view the content of a digital radio transmission only at the time the content is being broadcast and only in the location where the user has physically placed the digital radio receiver. Not only is the user of a digital radio receiver limited in terms of the location in which he may listen to or view digital radio content, but he may only hear or view a portion of the available content depending on the time when he tunes his receiver to the selected channel. These shortcomings can be addressed by exploiting the advantages presented by digital radio's use of a digital, rather than analog, signal, namely, the ease of transmitting and storing digital information or content.
With the development of digital transmission and storage of music, video and other content, the owners of such content have become increasingly concerned with copyright infringement for several reasons. In a digital environment, content can be transmitted in digital quality from one entity to the next without any degradation in quality. Therefore, a need has arisen for a system which exploits the advantages of digital radio in conjunction with digital storage devices, while providing adequate protection for the copyright interests of digital radio content providers.